Compensation and calibration for a low power bio-impedance measurement device

ABSTRACT

A method and apparatus for compensating and calibrating a bio-impedance measurement device are provided. In the method and apparatus, a memory stores a plurality of compensation parameters and a first detection channel receives a first detection signal, compensates the first detection signal using a first compensation parameter of the plurality of compensation parameters. In the method and apparatus, a second detection channel receives a second detection signal and a third detection signal and compensates the second and third detection signals using second and third compensation parameters of the plurality of compensation parameters and the compensated first detection signal. The impedance measurement device generates a first output signal representative of a first impedance measurement and a second output signal representative of a second impedance measurement based on the compensated first, second and third detection signals.

BACKGROUND Technical Field

This application is directed to a bio-impedance measurement device thatis calibrated to compensate the inaccuracies of the impedancemeasurements.

Description of the Related Art

Bio-impedance measurement has a wide range of applications.Bio-impedance may be used to determine the composition of a biologicalbody. Bio-impedance may also be used to determine the cardiac output ofthe biological body and its breathing rate. An accurate bio-impedancemeasurement aids in accurately characterizing the conditions of thebiological body. However, conventional bio-impedance measurement devicesintroduce errors that result in inaccurate bio-impedance measurements.

Wearable and portable devices have low power consumption requirements,such as the architectures described in U.S. Pat. Nos. 8,909,333 and9,307,924 and U.S. Patent Application Publication Nos. 2013/0006136 and2015/0051505. However, when working frequency is increased, the accuracyof these solutions is degraded. Therefore, for a multi-frequency (orhigh single frequency) device, novel architectural solutions are neededto enable high accuracy with low power consumption. It is desirable tocompensate for the errors and inaccuracies introduced in thebio-impedance measurements made by a bio-impedance measurement device.

BRIEF SUMMARY

In an embodiment, an impedance measurement device includes memoryconfigured to store a plurality of compensation parameters and a firstdetection channel configured to receive a first detection signal andcompensate the first detection signal using a first compensationparameter of the plurality of compensation parameters. The impedancemeasurement device also includes a second detection channel configuredto receive a second detection signal and a third detection signal andcompensate the second and third detection signals using second and thirdcompensation parameters of the plurality of compensation parameters andthe compensated first detection signal. The impedance measurement devicegenerates a first output signal representative of a first impedancemeasurement and a second output signal representative of a secondimpedance measurement based on the compensated first, second and thirddetection signals.

In an embodiment, the first and second detection channels are configuredto compensate for a relative time quantization error introduced in atrigger signal used for sampling the first detection signal and thesecond and third detection signals. In an embodiment, compensating thefirst detection signal using the first compensation parameter includesscaling an amplitude and adjusting a phase of the first detection signalby the first compensation parameter. In an embodiment, the firstcompensation parameter is a complex value. In an embodiment, generatingthe first output signal includes demodulating the compensated firstdetection signal to produce a first demodulated signal, filtering thefirst demodulated signal and compensating the filtered first demodulatedsignal to produce the first output signal.

In an embodiment, compensating the second and the third detectionsignals using the second and the third compensation parameters includesdetermining a difference between the second and the third detectionsignals, compensating the difference between the second and the thirddetection signals by the second compensation parameter, determining acommon mode voltage based on the compensated difference between thesecond and the third detection signals and the compensated firstdetection signal, compensating the common mode voltage by the thirdcompensation parameter and reducing the scaled difference between thesecond and the third detection signals by the scaled common modevoltage.

In an embodiment, generating the second output signal includesdemodulating the second and third detection signals, amplifying thedifference between the second and the third detection signals to producean amplified signal, filtering the amplified signal and compensating thefiltered amplified signal to produce the second output signal. In anembodiment, the first detection channel includes a first demodulator andthe first compensation parameter compensates for a gain of the firstdemodulator and an absolute time quantization error of the firstdemodulator.

In an embodiment, the second detection channel includes a seconddemodulator, a third demodulator and an amplifier and the secondcompensation parameter compensates for a gain of the second demodulatoror the third demodulator and an absolute time quantization error of thesecond demodulator or the third demodulator and a gain of the amplifier.In an embodiment, the second detection channel is configured to use thethird compensation parameter to compensate for a common mode rejectionratio of the second demodulator and the third demodulator.

In an embodiment, a method for calibrating an impedance measurementdevice includes setting a contact impedance of a plurality of probes ofthe impedance measurement device to a first impedance value and animpedance between two probes of the plurality of probes to a zeroimpedance value. In an embodiment, the method includes determining afirst detection signal at an input of a first detection channel of theimpedance measurement device and a first output signal at an output ofthe first detection channel. In an embodiment, the method includesdetermining a first compensation parameter based on the first detectionsignal and the first output signal. In an embodiment, the methodincludes detecting a second and a third detection signals at respectivefirst and second inputs of a second detection channel of the impedancemeasurement device and a second output signal at an output of the seconddetection channel.

In an embodiment, the method includes compensating, in the first andsecond detection channels, for a relative time quantization errorintroduced in a trigger signal used for sampling the first, second andthird detection signals. In an embodiment, the method includes setting acontact impedance of a plurality of probes of the impedance measurementdevice to a zero impedance value and an impedance between two probes ofthe plurality of probes to a second impedance value. In an embodiment,the method includes determining fourth and fifth detection signals atthe respective first and second inputs of the second detection channeland a third output signal at the output of the second detection channel.In an embodiment, the method includes determining second and thirdcompensation parameters based on the second, third, fourth and fifthdetection signals and the second and third output signals. In anembodiment, the method includes causing the first, second and thirdcompensation parameters to be stored in the impedance measurement devicefor compensating an impedance measurement to be made by the impedancemeasurement device.

In an embodiment, the method includes receiving a first detectionsignal, compensating the first detection signal using a firstcompensation parameter of the plurality of compensation parameters,receiving a second detection signal and a third detection signal,compensating the second and third detection signals using second andthird compensation parameters of the plurality of compensationparameters and the compensated first detection signal and generating afirst output signal representative of a first impedance measurement anda second output signal representative of a second impedance measurementbased on the compensated first, second and third detection signals.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a bio-impedance measurement device in contact with abiological body.

FIG. 2 shows a block diagram of the measurement device.

FIG. 3 shows a schematic of a circuit that models capacitances at aninput of the first and second detection channels of the measurementdevice.

FIGS. 4A and 4B show a block diagram of a method for determining thecompensation parameters of the measurement device.

FIG. 5 shows a block diagram of a method for measuring impedance.

FIG. 6 shows an example of a waveform generated by an accumulator.

FIG. 7 shows a calibration device coupled to the measurement device.

DETAILED DESCRIPTION

FIG. 1 shows a bio-impedance measurement device 100 in contact with abiological body 102. The bio-impedance measurement device 100, referredto hereinafter as the measurement device 100, includes a plurality ofelectrodes 104 a-104 d (collectively referred to herein as electrodes104), a current generator 106 and a voltage detector 108. The currentgenerator 106 is coupled to a first electrode 104 a and a secondelectrode 104 b of the plurality of electrodes 104. The voltage detector108 is coupled to a third electrode 104 c and a fourth electrode 104 dof the plurality of electrodes 104.

The plurality of electrodes 104 make contact with the biological body102. For example, each electrode 104 may be positioned to be in contactwith the skin or tissue of the biological body 102. The measurementdevice 100 may be any device that measures the impedance (also known asthe bio-impedance or bioelectrical impedance) of an object. Impedance isa measure of the opposition to current by the object. The impedance ofthe biological body 102 may be indicative of the composition of thebiological body 102. For example, the impedance of the biological body102 may be used to determine an amount of water or liquids in thebiological body, fat-free body mass, or body fat.

The current generator 106 supplies current between the first and secondelectrodes 104 a, 104 b. The voltage detector 108 measures the impedanceacross the third and fourth electrodes 104 c, 104 d. Measuring theimpedance may be based on a voltage detected at the third and fourthelectrodes 104 c, 104 d. As described herein, the measurement device 100may also measure the impedance at the first and second electrodes 104 a,104 b and utilize the measurement to improve impedance detection.

The measurement device 100 may be a wearable device. For example, themeasurement device 100 may be part of a watch, an activity tracker, anarmband, a chest band or a patch, among others. The measurement device100 may be used to provide biometrics, for example, to a user. Thebiometrics may include body composition or fluid content among others.

FIG. 2 shows a block diagram of the measurement device 100. Themeasurement device 100 includes the plurality of electrodes 104, thecurrent generator 106, the voltage detector 108, a controller 110 and atimer 112. The voltage detector 108 includes a first detection channel114 and a second detection channel 116. The first detection channel 114includes a first demodulator 118, a first filter 120 and a firstanalog-to-digital converter (ADC) 122. The second detection channel 116includes a second demodulator 124, a third demodulator 126, andamplifier 128, a second filter 130 and a second ADC 132. The first andsecond filters 120, 130 may be low-pass filters. The current generatorincludes a waveform generator 134 and a voltage-to-current converter136. The measurement device 100 also includes memory 137. It is notedthat in an embodiment, the first and second ADCs 122, 132 may beintegrated into the controller 110. Furthermore, the controller 110 mayperform the functions of the first and second ADCs 122, 132.

The measurement device 100 reduces power consumption. In particular theuse of the demodulators before the amplification in the channels 116,114 enables the use of a narrower bandwidth amplifier. Thus reduces bothpower consumption and device cost. Furthermore, the detection channel114 may use a voltage output of the voltage-to-current converter 136 tomeasure the impedance of the body (Z0) plus the electrode-to-skincontact impedances.

The controller 110 sets the frequency for operating waveform generator134 and the demodulators 118, 124, 126. The controller 110 outputs aconfiguration signal indicating the frequency. The waveform generator134 receives the configuration signal and generates a waveform havingthe frequency. The waveform may, for example, be a sine waveform amongothers. The waveform generator 134 outputs the waveform to thevoltage-to-current converter 136.

The waveform generator 134 outputs a synchronization signal to the timer112. The timer 112 receives the synchronization signal from the waveformgenerator 134 and also receives the configuration signal from thecontroller 110. The timer 112 outputs a trigger signal 113 to thedemodulators 118, 124, 126. The demodulators 118, 124, 126 synchronouslysample the detected voltage, based on the trigger signal, generatingvoltage signals proportional to the real and imaginary parts of theimpedances.

The voltage-to-current converter 136 receives the waveform from thewaveform generator 134. The voltage-to-current converter 136 convertsthe waveform from a voltage signal to a current signal. Thevoltage-to-current converter 136 outputs of the current signal over thefirst electrode 104 a. The second electrode 104 b may be connected tothe ground. The current is supplied to the biological body 102 betweenthe first and second electrodes 104 a, 104 b.

The first detection channel 114 has an input coupled to the firstelectrode 104 a. The input may be a voltage signal generated by thevoltage-to-current converter 136. The voltage signal may be proportionalto the voltage drop between the electrodes 104 a, 104 b. The inputsignal is described in U.S. Pat. No. 8,909,333. The second detectionchannel 116 has a first input coupled to the third electrode 104 c and asecond input coupled to the fourth electrode 104 d. The first detectionchannel 114 operates on an input received from the first electrode 104 aand produces a first filtered signal 148 (V_(INJ,3)). The seconddetection channel 116 operates on inputs received at the third andfourth electrodes 104 c, 104 d. The second detection channel 116produces a second filtered signal 160 (V_(0,4)). The first filteredsignal 148 and the second filtered signal 160 are then used to determinea measurement of a contact impedance (Z_(E)) (denoted as contactimpedance 140) and a measurement of the bio-impedance (Z₀) (denoted asbio-impedance 142) of the biological body 102 as described herein. Thecontact impedance 140 is the impedance made between one of theelectrodes 104 and the biological body 102.

In particular, the first demodulator 118 receives a first detectionsignal 144 that is output from the first electrode 104 a. The firstdemodulator 118 demodulates the first detection signal 114 based on thetrigger signal 113 and outputs a first demodulated signal 146. The firstdemodulated signal 146 is provided to the first filter 120. The firstfilter 120 receives the first demodulated signal 146 and filters thedemodulated signal 146. The first filter 120 may low-pass filter thefirst demodulated signal 146. The first filter 120 outputs a firstfiltered signal 148. The first filtered signal 148 may be converted fromanalog to digital format by the first analog-to-digital converter 122.

In the second detection channel 116, the second demodulator 124 receivesa second detection signal 150 that is output from the third electrode104 c. The third demodulator 126 receives a third detection signal 152that is output from the fourth electrode 104 d. The second demodulator124 demodulates the second detection signal 150 based on a timing of thetrigger signal 113. The second demodulator 124 outputs a seconddemodulated signal 154. The third demodulator 126 demodulates the thirddetection signal 152 based on a timing of the trigger signal 113. Thethird demodulator 126 outputs a third demodulated signal 156.

The amplifier 128 receives the second and third demodulated signals 154,156. The amplifier 128 compares the second and third demodulated signals154, 156. The amplifier 128 outputs an amplified signal 158 based on adifference between the third demodulated signal 156 and the seconddemodulated signal 154. The second filter 130 receives the amplifiedsignal 158. The second filter 130 filters the amplified signal 158 toproduce the second filtered signal 160. The second filter 130 may be alow-pass filter and may remove, from the amplified signal 158, frequencycomponents that are higher than a threshold frequency. The secondfiltered signal 160 may be converted from analog format to digitalformat by the second analog-to-digital converter 132.

The first filtered signal 160 and the second filtered signal 148 maythen be used to determine the detected contact impedance 140 and thedetected bio-impedance 142 as described herein.

The memory 137 is configured to store compensation parameters. Thecompensation parameters are used to compensate the output signals of thefirst and second ADCs 122, 132.

The accuracy of measuring the bio-impedance 142 by the measurementdevice 100 is affected by several factors. The factors include an inputparasitic impedance, e.g. a parasitic capacitance, to the demodulatorsand a gain accuracy of the demodulators 118, 124, 126. The factors alsoinclude a common mode rejection ratio of the second and thirddemodulators 124, 126. The factors affecting accuracy of thebio-impedance 142 measurement include the clock granularity of thetrigger signal 113 and a delay associated with many blocks of thecircuit, e.g. the trigger signal 113 the demodulators 118, 154, 158, thevoltage-to-current converter 136, etc. Further, the accuracy of thewaveform generator 134 also influences the accuracy of measuring thebio-impedance 142. These error sources become especially critical whenthe power consumption is reduced. Low power consumption, i.e., requiresthe use of limited clock speed and of components with bandwidth asnarrow as possible. Therefore, limited performances, like reduced CMRR,not negligible parasitic, lower clock granularity shall be expected,especially for high working frequency of the system.

As described herein, the measurement device 100 is calibrated tocompensate for the factors that introduce inaccuracies in thebio-impedance measurement. After the measurement device is calibrated,the accuracy of the bio-impedance measurement is improved. Themeasurement device 100 may be calibrated during or after manufacture.The measurement device 100 may be configured to compensate for thesources of inaccuracy. When the measurement device 100 is used, theaccuracy of bio-impedance measurement is improved as a result of thecalibration and compensation of the sources of inaccuracy.

The measurement device 100 may be calibrated by applying a known contactimpedance 140 and a known bio-impedance 142. For example, themeasurement device may be used to measure the known impedances 140, 142.Thereafter, a plurality of compensation parameters may be obtained. Themeasurement device 100 may then be configured to compensate for theplurality of measured compensation parameters. Calibration may beperformed during or after manufacture of the measurement device 100.Parameter compensation may be performed while the measurement device 100is being used, for example, by a user.

FIG. 3 shows a schematic of a circuit that models capacitances at inputsof the first and second detection channels 114, 116 of the measurementdevice 100. The first detection channel 114 receives an injected voltage(V_(INJ,1)). The second detection channel 116 receives a first voltage(V_(P1)) and a second voltage (V_(P2)). The injected voltage is avoltage of a voltage injection node 202. The first and second voltagesare voltages of a first node 204 and a second node 206, respectively.

A voltage source 208 has a cathode coupled to the voltage injection node202 and an anode coupled to a ground node 210. A first impedance, e.g. acapacitance 212 (denoted as C₁) may be coupled in series to the contactimpedance 140 for safety reasons, preventing DC current to flow in thebody. The first capacitance 212 and the contact impedance 140 aretogether coupled between the voltage injection node 202 and the firstnode 204.

A second impedance, e.g. a second parasitic capacitance 214 (denoted asC_(s1)) is coupled between the first node 204 and the ground node 210.The bio-impedance 142 is coupled between the first and second nodes 204,206. A third impedance, e.g. a third parasitic capacitance 216 (denotedas C_(s2)) is coupled between the second node 206 and the ground node210. The value of C_(s1) and C_(s2) may be determined, e.g., throughdatasheet of components, simulations or measurements. Another contactimpedance 140 is coupled between the second node 206 and the ground node210. A current source 218 that provides an injected current (I_(INJ))has an anode coupled to the contact impedance 140 and a cathode coupledto a virtual ground node 211. The virtual ground node 211 may be a nodethat is kept to the same voltage as a ground node, but current cannotflow into the virtual ground node 211

The current generated by the current source 218 is partially absorbed bythe capacitances 212, 214, 216. Due to the capacitances 212, 214, 216,the injection voltage (V_(INJ,1)) received by the first detectionchannel 114 and the first voltage (V_(P1)) and the second voltage(V_(P2)) received by the second detection channel 116 are changed.

The injection voltage is a function of the bio-impedance 142, contactimpedance 140, first, second and third capacitances 212, 214, 216 andinjection current. The injection voltage may, therefore, be representedas:

V _(INJ,1) =f ₁(Z ₀ , Z _(E) , C _(s1) , C _(s2) , C ₁ , I _(INJ)).  Equation (1)

Similarly, the first voltage and the second voltage are functions of thebio-impedance 142, contact impedance 140, first, second and thirdcapacitances 212, 214, 214 and injection current. In addition, thedifference between the first voltage and the second voltage is afunction of the quantities and may be represented as:

V _(d,1) =V _(P1) −V _(P2) =f ₂(Z ₀ , Z _(E) , C _(s1) , C _(s2) , C ₁ ,I _(INJ)).   Equation (2)

The common mode voltage, which is the average of the first and secondvoltages, may be represented as:

$\begin{matrix}{V_{CM} = {\frac{V_{P\; 1} + V_{P\; 2}}{2} = {{f_{3}\left( {Z_{0},Z_{E},C_{s\; 1},C_{s\; 2},C_{1},I_{INJ}} \right)}.}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

The bio-impedance 142 is function of the injection voltage 208, thedifference between the first voltage 204 and the second voltage 206, thefirst, second and third capacitances 212, 214, 214 and injectioncurrent:

Z ₀ =f ₄(V _(d,1) , V _(INJ,1) , C _(s1) , C _(s2) , C ₇ , I _(INJ)).  Equation (4a)

The contact impedance 140 is function of the injection voltage 208 thedifference between the first voltage 204 and the second voltage 206, thefirst, second and third capacitances 212, 214, 214 and injectioncurrent:

Z _(E) =f ₅(V _(d,1) , V _(INJ,1) , C _(s1) , C _(s2) , C ₇ , I _(INJ)).  Equation (4b)

Similarly, also the common mode voltage can be represented as a functionof the injection voltage 208, the difference between the first voltage204 and the second voltage 206, the first, second and third capacitances212, 214, 214 and injection current:

V _(CM) =f ₆(V _(d,1) , V _(INJ,1) , C _(s1) , C _(s2) , C ₇ , I_(INJ)).   Equation (4c)

Any skilled person can compute the expression of f₁, f₂, f₃, f₄, f₅ andf₆ from the analysis of the circuit in FIG. 3.

Referring back to FIG. 2, the first detection signal 144 (denoted asV_(INJ,1)) received by the first demodulator 118 is the same as theinjection voltage. The first demodulated signal 146 that is output bythe first demodulator may be represented as:

V _(INJ,2) =G _(D,I) V _(INJ,1) e ^(jT) ^(D,I) ,   Equation (4d)

where G_(D,I) represents the gain of the first demodulator 118, T_(D,I)represents a first time delay and j represents the unit imaginarynumber.

The first filtered signal 148 may also be represented as:

$\begin{matrix}{{V_{{INJ},3} = V_{{INJ},2}}{V_{{INJ},3} = {\left( {G_{D,I}e^{{jT}_{D,I}}} \right)V_{{INJ},1}}}{{V_{{INJ},3}\overset{def}{=}{\frac{1}{A_{E}}V_{{INJ},1}}},}} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

where A_(E) is a compensation parameter associated with the firstdetection channel 114. It is noted that V_(INJ,3)=V_(INJ,2) is true inthe passband of the filter 120.

In the second detection channel 116, the difference between the seconddetection signal 150 and the third detection signal 152 is given byEquation (2). As described herein, the second and third demodulators124, 126 respectively demodulate the second and third detection signals150, 152 and output the second and third demodulated signals 154, 156.The difference between the second and third demodulated signals 154, 156is denoted herein as V_(d,2) and may be represented as:

V _(d,2) =G _(D,0)(V _(d,1)+CMRR_(FD) V _(CM))e ^(jT) ^(D,0) ,  Equation (6)

where G_(D,0) represents the gain of the second and third demodulators124, 126, CMRR_(FD) is the common mode rejection ratio of the of thesecond and third demodulators 124, 126, V_(CM) is the common modevoltage described with reference to Equation (3) and T_(D,0) is a secondtime delay.

The amplified signal 158 output by the amplifier 128 may be representedas:

V _(0,3) =G _(A) V _(d,2) V _(0,3) =G _(A) G _(D,0)(V _(d,1)+CMRR_(FD) V_(CM))e ^(jT) ^(D,0) ,   Equation (7)

where G_(A) is the gain of the amplifier 158.

The second filtered signal 160 (V_(0,4)) output by the second filter 130may be assumed to be the same as the amplified signal 158. That is,V_(0,4) may be set to V_(0,3). The second filtered signal 160 may berepresented as:

V _(0,4) =G _(A) G _(D,0)(V _(d,1)+CMRR_(FD) V _(CM))e ^(jT) ^(D,0) V_(0,4)=(V _(d,1)+CMRR_(FD) V _(CM))(G _(A) G _(D,0) e ^(jT) ^(D,0) .  Equation (8)

The second part of Equation (8) may be defined to be a firstcompensation parameter of the second detection channel 116. The firstcompensation parameter may be defined as:

$\begin{matrix}{\frac{1}{A_{0}}\overset{def}{=}{\left( {G_{A}G_{D,0}e^{{jT}_{D,0}}} \right).}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

The common mode rejection ratio (CMRR_(FD)) may be defined as a secondcompensation parameter of the second detection channel 116. Thus, thesecond filtered signal output by the second filter 130 may berepresented as

$\begin{matrix}{V_{0,4} = {\frac{1}{A_{0}}{\left( {V_{d,1} + {{CMRR}_{FD}V_{CM}}} \right).}}} & {{Equation}\mspace{14mu} (10)}\end{matrix}$

FIGS. 4A and 4B show a block diagram of a method 400 for determining thecompensation parameters of the measurement device 100. The method 400may be performed by a calibration device connected to the measurementdevice 100. At step 402, the calibration device sets the contactimpedance 140 is to zero Ω and the bio-impedance to a fixed impedance.At step 404, the calibration device sets the input frequency of theinjection current at a frequency value. At step 406, the calibrationdevice determines second filtered signal 160 (denoted as {circumflexover (V)}_(dm1)). The second filtered signal 160 is provided at theoutput of the second filter 130. To determine the second filteredsignal, the calibration device may be connected to an output of thesecond filter 130.

At step 408, the calibration device determines a first differencebetween the second detection signal 150 and the third detection signal152, for example using the Equation (2). The first difference is denotedherein as {circumflex over (V)}_(dt1). The calibration device calculatesthe second and third detection signals 150, 152 (and their difference)using Equation (2). That is because the capacitances and the current areknown, while the Z0 and ZE are used for the calibration.

The calibration device then determines a first common mode voltage atstep 410 (denoted herein as {circumflex over (V)}_(CM1)). Thecalibration device may determine the first common mode voltage, forexample, using Equation (3).

The method 400 proceeds to step 412. At step 412, the calibration devicesets the bio-impedance 142 to zero Ω and the contact impedance 140 ofelectrodes 104 to a fixed impedance. At step 414, the calibration devicesets the input frequency of the injection current at a frequency value.At step 416, the calibration device determines the second filteredsignal 160 detected at zero Ω bio-impedance and a fixed contactimpedance 140. The second filtered signal 160 is denoted herein as{circumflex over (V)}_(dm2). At step 418, the calibration devicedetermines a second difference between the second detection signal 150and the third detection signal 152 detected at zero Ω bio-impedance anda fixed contact impedance 140. The second difference between the seconddetection signal 150 and the third detection signal 152 is denotedherein as {circumflex over (V)}_(dt2). The calibration device thendetermines a second common mode voltage (denoted as {circumflex over(V)}_(CM2)) at step 420.

At step 422, the calibration devices uses the second filtered signal 160({circumflex over (V)}_(dm1)), the first difference signal ({circumflexover (V)}_(dt1)) and the first common mode voltage ({circumflex over(V)}_(CM1)) determined at the zero contact impedance and fixedbio-impedance as well as the second filtered signal 160 ({circumflexover (V)}_(dm2)), the second difference signal ({circumflex over(V)}_(dt2)) and the second common mode voltage ({circumflex over(V)}_(CM2)) determined at the fixed contact impedance 140 and zerobio-impedance 142 to determine the first and second compensationparameters (A₀ and CMRR_(FD)) of the second detection channel. Todetermine the first and second compensation parameters, the calibrationdevice uses Equation (10). In particular, when the contact impedance iszero and the bio-impedance is fixed to an impedance value, Equation (10)may be represented as:

{circumflex over (V)} _(dm1) A ₀ ={circumflex over (V)} _(dt1)+{circumflex over (V)} _(CM1)CMRR_(FD).   Equation (11)

When the contact impedance 140 is set to an impedance value and thebio-impedance is zero, Equation (10) may be represented as:

{circumflex over (V)} _(dm2) A ₀ ={circumflex over (V)} _(dt2)+{circumflex over (V)} _(CM2)CMRR_(FD).   Equation (12)

Having two unknowns, Equations (11) and (12) may be solved together toobtain the first and second compensation parameters.

After setting the injection frequency at step 414, the calibrationdevice determines the compensation parameter of the first detectionchannel. At step 424, the calibration device determines the firstdetection signal 144 (denoted herein as {circumflex over (V)}_(INJ,t)),for example using Equation (1). At step 426, the calibration devicedetermines the first filtered signal 148 that is output by the firstfilter 120 and denoted herein as {circumflex over (V)}_(INJ,m). At step428, the calibration device determines the compensation parameter(A_(E)) of the first detection channel based on the first detectionsignal 144 and the first filtered signal 148. The calibration device maydetermine the compensation parameter (A_(E)) using Equation (5). Inparticular, the compensation parameter of the first detection channelmay be determined as:

{circumflex over (V)}_(INJ,m)A_(E)={circumflex over (V)}_(INJ,t).  Equation (13)

The calibration device, at step 430, causes the compensation parameterof the first detection channel and the first and second compensationparameters of the second detection channel to be stored in themeasurement device 100. The procedure can be repeated for differentfrequencies. The measurement device 100 utilizes the compensationparameters as described herein to improve the obtained bio-impedance andcontact impedance measurements.

FIG. 5 shows a block diagram of a method 500 for measuring impedance.The method 500 may be performed by the measurement device 100. At step502, the measurement device 100 begins an impedance measurement. Toperform the impedance measurement, the measurement device 100 may beconnected to a biological body 102 as described with reference to FIG. 1herein, or through a different disposition of the 4 electrodes.

At step 504, the measurement device 100 determines a difference betweenthe second and third detection signals 150, 152. The difference betweenthe second and third detection signals 150, 152 is denoted herein as{circumflex over (V)}_(dm). At step 506, the measurement device 100determines the first detection signal 148 (denoted herein as {circumflexover (V)}_(INJ,m)).

At step 508, the measurement device 100 compensates the first detectionsignal using the compensation parameter of the first detection channel.To compensate the first detection signal, the measurement device 100 mayscale the first detection signal by the compensation parameter of thefirst detection channel as:

{circumflex over (V)}_(INJ,C)={circumflex over (V)}_(INJ,m)A_(E).  Equation (14)

At step 510, the measurement device 100 compensates the differencebetween the second and third detection signals using the firstcompensation parameter of the second detection channel. The measurementdevice 100 may compensate the difference between the second and thirddetection signals by scaling the difference by the first compensationparameter of the second detection channel as:

{circumflex over (V)}_(dC1)={circumflex over (V)}_(dm)A₀.   Equation(15)

At step 512, the measurement device 100 determines the common modevoltage. The common mode voltage (denoted as {circumflex over (V)}_(CM))may be obtained from Equation (4c) assuming V_(d,1)≈{circumflex over(V)}_(d). At step 514, the measurement device 100 compensates thedifference between the second and third detection signals using thecommon mode voltage and the second compensation parameter of the seconddetection channel. The compensation performed at step 514 operates onthe compensated difference performed at step 510. In an embodiment, theaccuracy of the determination of the common mode voltage at step 512 maybe improved through iteration of steps 512 and 514.

Compensating the difference between the second and third detectionsignals includes removing the contribution of common mode rejection fromthe difference between the second and third detection signals. Thedifference between the second and third detection signals may becompensated as:

{circumflex over (V)} _(dC2) ={circumflex over (V)} _(dC1)−CMRR_(FD){circumflex over (V)} _(CM).   Equation (16)

The measurement device 100, at step 516, determines the bio-impedancebased at least in part on the compensated difference between the secondand third detection signals and the compensated first detection signal.This may be done using Equation (4a). The measurement device 100, atstep 518, determines the contact impedance based at least in part on thecompensated difference between the second and third detection signalsand the compensated first detection signal. This may be done usingEquation (4b).

In an embodiment, the waveform generator 134 may be an accumulatorregister and may be incremented by a fixed value each clock cycle. Whenthe incremented sum, exceeds a maximum value that may be stored in theregister, the incremented sum wraps around (for example, as a result ofa modulo operation).

FIG. 6 shows an example of a waveform generated by an accumulator. Theaccumulator generates a sawtooth-like envelope for sine wave peaks andvalleys that is in effect a sum of a sine wave and a sawtooth. Followingan envelope “tooth,” the accumulator output may be different from aprevious cycle. Line 602 shows a first-order envelope of the accumulatoroutput and line 604 shows a second-order envelope of the accumulatoroutput.

The trigger signal 113 may follow the first-order envelope, thesecond-order envelope or an envelope of another order. If the frequencyof all the resulting envelopes in the trigger signal 113 is large enoughto be outside the acquisition signal bandwidth and filtered by theacquisition stage then the effect of the varying output of theaccumulator is minimized. This may be taken into account before to setthe frequency in the blocks 404, 414 (FIGS. 4A and 4B) and 502 (FIG. 5).

The frequency of the injected current may be significantly smaller thanthe clock frequency of the trigger signal 113. If the injected currentfrequency is not significantly smaller than the clock frequency of thetrigger signal 113, the sample and hold trigger time quantization of thedemodulators 118,124, 126 may not be negligible. Consequently, thedetection signals 114, 150, 152 may not be sampled by the respectivedemodulators 118,124, 126 at a precise phase (for example, 0° and 90°for phase-quadrature demodulation). The resulting absolute timequantization error that affects the sampling triggers is modeled asdelays (taken into account in T_(D,I) and T_(D,0)) herein. The absolutetime quantization errors are compensated by A_(E) and A₀.

Further, another error (relative error) may be introduced betweenconsecutive triggers of the trigger signal (for example, the timedifference between a 0° trigger and a 90° trigger). Geometrically, therelative error may be modeled as a projection on two non-orthogonal axes(the ideal phase-quadrature demodulation is a projection of a sine waveon two orthogonal axes). The angle of deviation from orthogonality maybe compensated in each detection signal 148 and 160. The compensationmay be executed, immediately after the ADC conversion, in blocks 406,416 and 426 (FIGS. 4A and 4B) and 504 and 506 (FIG. 5).

In low-power application, a slow clock frequency is used and granularitymay be poor. Further, contrary to the absolute time quantization error,which results in a delay and which is taken into account in the formulasabove, the relative error parameter may not be a delay or a gain error.The relative error parameter may be compensated during operation (alsoduring calibration) after ADC conversion.

FIG. 7 shows a calibration device 101 coupled to the measurement device100. The calibration device 101 is used for determining the compensationparameters as described herein. The calibration device 101 is connectedto the outputs of the ADCs 122, 132 and receives the converted signalsfrom the ADCs 122, 132. The calibration device 101 is coupled to thecontroller 110, whereby the calibration device 101 may instruct thecontroller 110 to perform a frequency sweep for the injection current.The calibration device 101 is coupled to the memory 137.

The calibration device 101 may detect the converted filtered signals148, 160 respectively output by the ADCs 122, 132. The calibrationdevice 101 may use the detected signals to determine the compensationparameters as described herein.

The calibration device 101 may store the compensation parameters in thememory 137 of the measurement device 100. During use, the measurementdevice 100 may use the compensation parameters stored in the memory 137to compensate for sources of error in the impedance measurements asdescribed herein.

In an embodiment, the first detection channel 114 may be replicated. Thedetection channel replica may be configured to receive the thirddetection signal 152, for example, in the event that the two contactimpedances 140 described with reference to FIG. 3 are different.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. An impedance measurement device, comprising: memory configured tostore a plurality of compensation parameters; a first detection channelconfigured to: receive a first detection signal; and compensate thefirst detection signal using a first compensation parameter of theplurality of compensation parameters; and a second detection channelconfigured to: receive a second detection signal and a third detectionsignal; and compensate the second and third detection signals usingsecond and third compensation parameters of the plurality ofcompensation parameters and the compensated first detection signal;wherein the impedance measurement device generates a first output signalrepresentative of a first impedance measurement and a second outputsignal representative of a second impedance measurement based on thecompensated first, second and third detection signals.
 2. The impedancemeasurement device of claim 1, wherein the first and second detectionchannels are configured to compensate for a relative time quantizationerror introduced in a trigger signal used for sampling the firstdetection signal and the second and third detection signals.
 3. Theimpedance measurement device of claim 1, wherein compensating the firstdetection signal using the first compensation parameter includes scalingan amplitude and adjusting a phase of the first detection signal by thefirst compensation parameter, wherein the first compensation parameteris a complex value.
 4. The impedance measurement device of claim 1,wherein generating the first output signal includes: demodulating thecompensated first detection signal to produce a first demodulatedsignal; filtering the first demodulated signal; and compensating thefiltered first demodulated signal to produce the first output signal. 5.The impedance measurement device of claim 1, wherein compensating thesecond and the third detection signals using the second and the thirdcompensation parameters includes: determining a difference between thesecond and the third detection signals; compensating the differencebetween the second and the third detection signals by the secondcompensation parameter; determining a common mode voltage based on thecompensated difference between the second and the third detectionsignals and the compensated first detection signal; compensating thecommon mode voltage by the third compensation parameter; and reducingthe scaled difference between the second and the third detection signalsby the scaled common mode voltage.
 6. The impedance measurement deviceof claim 5, wherein generating the second output signal includes:demodulating the second and third detection signals; amplifying thedifference between the second and the third detection signals to producean amplified signal; filtering the amplified signal; and compensatingthe filtered amplified signal to produce the second output signal. 7.The impedance measurement device of claim 1, wherein the first detectionchannel includes a first demodulator and the first compensationparameter compensates for a gain of the first demodulator and anabsolute time quantization error of the first demodulator.
 8. Theimpedance measurement device of claim 1, wherein the second detectionchannel includes a second demodulator, a third demodulator and anamplifier and the second compensation parameter compensates for a gainof the second demodulator or the third demodulator and an absolute timequantization error of the second demodulator or the third demodulatorand a gain of the amplifier.
 9. The impedance measurement device ofclaim 8, wherein the second detection channel is configured to use thethird compensation parameter to compensate for a common mode rejectionratio of the second demodulator and the third demodulator. 10-13.(canceled)
 14. A method of measuring impedance, the method comprising:using a first detection channel, receiving a first detection signal;using the first detection channel, compensating the first detectionsignal using a first compensation parameter of the plurality ofcompensation parameters; using a second detection channel, receiving asecond detection signal and a third detection signal; and using thesecond detection channel, compensating the second and third detectionsignals using second and third compensation parameters of the pluralityof compensation parameters and the compensated first detection signal;and generating a first output signal representative of a first impedancemeasurement and a second output signal representative of a secondimpedance measurement based on the compensated first, second and thirddetection signals.
 15. The method of claim 14, comprising: compensatingthe first, second and third detection signals for a relative timequantization error introduced in a trigger signal used for sampling thefirst detection signal and the second and third detection signals. 16.The method of claim 14, wherein compensating the first detection signalusing the first compensation parameter includes scaling an amplitude andadjusting a phase of the first detection signal by the firstcompensation parameter, wherein the first compensation parameter is acomplex value.
 17. The method of claim 14, wherein generating the firstoutput signal includes: demodulating the first detection signal toproduce a first demodulated signal; filtering the first demodulatedsignal; and compensating the filtered first demodulated signal toproduce the first output signal.
 18. The method of claim 14, whereincompensating the second and the third detection signals using the secondand the third compensation parameters includes: determining a differencebetween the second and the third detection signals; compensating thedifference between the second and the third detection signals by thesecond compensation parameter; determining a common mode voltage basedon the compensated difference between the second and the third detectionsignals and the compensated first detection signal; compensating thecommon mode voltage by the third compensation parameter; and reducingthe scaled difference between the second and the third detection signalsby the scaled common mode voltage.
 19. The method of claim 18, whereingenerating the second output signal includes: demodulating the secondand third detection signals; amplifying the difference between thesecond and the third detection signals to produce an amplified signal;filtering the amplified signal; and compensating the filtered amplifiedsignal to produce the second output signal.
 20. The method of claim 14,wherein the first detection channel includes a first demodulator and thefirst compensation parameter compensates for a gain of the firstdemodulator and an absolute time quantization error of the firstdemodulator.
 21. An impedance measurement device, comprising: aplurality of electrodes; a current generator configured to supplycurrent to the plurality of electrodes; memory configured to store aplurality of compensation parameters; and a voltage detector, thevoltage detector including a first detection channel and a seconddetection channel, wherein the first detection channel is configured toreceive a first detection signal and compensate the first detectionsignal using a first compensation parameter of the plurality ofcompensation parameters, wherein the second detection channel configuredto receive second and third detection signals and compensate the secondand third detection signals using second and third compensationparameters of the plurality of compensation parameters and thecompensated first detection signal, wherein the impedance measurementdevice generates a first output signal representative of a firstimpedance measurement and a second output signal representative of asecond impedance measurement based on the compensated first, second andthird detection signals.
 22. The impedance measurement device of claim21, wherein the impedance measurement device is part of a watch,activity tracker, chest band or patch.
 23. The impedance measurementdevice of claim 21, wherein the first and second detection channels areconfigured to compensate for a relative time quantization errorintroduced in a trigger signal used for sampling the first detectionsignal and the second and third detection signals.
 24. The impedancemeasurement device of claim 21, wherein compensating the first detectionsignal using the first compensation parameter includes scaling anamplitude and adjusting a phase of the first detection signal by thefirst compensation parameter, wherein the first compensation parameteris a complex value.